This project will address the need for real-time, multi-site, continuous measurement of oxygen partial pressure in vivo. A fabrication sequence for the construction of semiconductor based polarographic oxygen sensors will be developed. A set of masks will be used to fabricate isolated polarographic oxygen sensors for evaluation and optimization of the process and characterization of the sensors. The process will be incorporated into an integrated circuit fabrication sequence so that microelectronic circuits can be manufactured simultaneously on the same substrate as the oxygen sensors. These intelligent oxygen sensors will include a sensor, signal detection circuitry, and an analog to digital converter, and will be assembled into multisite, digital output probes. Each year over 1,000,000 new cases of cancer are reported in the United States alone; nearly 80 percent of those cases are treated with modalities for which oxygenation is a primary controlling factor. Radiotherapy relies on the interaction of the applied radiation with oxygen present in tissue to produce the cytotoxic free radicals O2- and HO-. Tissue oxygenation governs the selection of oxic or hypoxic chemotherapeutic agents. The sensors produced could also be used for a multitude of other applications, as tissue oxygenation is one of the most ubiquitous physiologic measurements.